Stormwater & Erosion Control
Runoff, infiltration and drainage sizing in one signed report, prepared to your agency's criteria for plan check.
A stormwater calculation package is the engineering analysis that estimates how much rain runs off a site and sizes the trenches, basins, Low Impact Development (LID) facilities and pipes that capture, treat or carry it for agency review.
Agencies may call it drainage calculations, a drainage report, hydrology study, or hydrology and hydraulics (H&H) calculations. It goes in with a grading, building or improvement permit.
It is not a SWPPP: a stormwater pollution prevention plan controls sediment and other pollutants during construction, while these calculations size permanent drainage.
Deliverables
Scope follows your agency's drainage manual. A typical package includes:
Tributary areas, flow paths and hard and landscaped surfaces mapped for each discharge point.
Peak flows and runoff volumes for the design storms your agency's drainage manual specifies.
LID facilities, leach trenches, basins and pipes sized, with capacity and drawdown checks.
Methods, inputs and results in one report, formatted for plan check or state permit filing.
Plan check asks for them when a project changes how water leaves the site.
Grading or new paving changes how a site drains. See grading plans.
Projects that reach their municipal permit's impervious-surface threshold need LID; plan it into the site layout.
Without a storm drain nearby, runoff may have to stay on site in trenches or basins.
Tentative maps and improvement plans often need tributary areas, pipe sizes and basin volumes.
Process
Five steps from site data to a sealed report.
We confirm the drainage manual, permit program and design storms, and gather the site plan, soils report and agency comments.
Drainage areas and flow paths are mapped from a topographic survey, and roof, paving and landscape areas are measured.
Peak flows and volumes use the agency's method and rainfall data: often the Rational Method (Q = CiA) for small areas, a unit hydrograph for larger ones.
Facilities are sized and checked for capacity, drawdown and hydraulic grade line, using measured infiltration rates where water soaks in.
Results feed the grading and improvement plans; the engineer in responsible charge seals the report and answers plan-check comments.
Most packages combine several of these.
A stormwater leach trench (infiltration trench) is a gravel-filled trench, often with perforated pipe, that holds runoff until it soaks in. Design volume, gravel voids and the infiltration rate at trench depth set its size. Septic leach lines are a separate septic design.
A retention basin holds runoff until it soaks in or evaporates; a detention basin stores it and releases it slowly through an outlet. Calculations set storage volume, outlet size, spillway and freeboard for local design storms.
LID calculations show how a project retains or treats runoff from new and replaced hard surfaces with bioretention, infiltration, harvesting or pervious paving. Under the statewide Small MS4 (municipal separate storm sewer system) permit, they cover:
Pipe sizing calculations confirm that storm drains, inlets and culverts carry the design flow: Manning's equation for capacity, and a hydraulic grade line check that water stays below grates and covers.
A tributary area is all the land, on site and off, that drains to one inlet, pipe, trench or basin; its surfaces set the runoff every other calculation uses.
California rules
State law, two statewide permits and federal injection-well rules set the baseline:
California law counts the design of drainage and flood-control works as civil engineering (§6731). A licensed civil engineer prepares the calculations or is in responsible charge of them, and signs, seals and dates the final set (§6735). Licensed architects practicing architecture are exempt (§6737).
Under the statewide Small MS4 permit (in effect as of September 2026), most projects creating or replacing 5,000 square feet or more of impervious surface must use LID, sized for roughly the 85th percentile 24-hour storm or a rainfall intensity of at least 0.2 inches per hour.
Except for linear underground and overhead projects, sites under the Construction General Permit upload post-construction plans and calculations to SMARTS, the state's online permit system. Where no local MS4 standard applies, the site must match its pre-construction runoff volume up to the 85th percentile 24-hour storm.
Perforated pipe that spreads stormwater underground is a subsurface fluid distribution system, which federal rules treat as a Class V injection well. EPA Region 9 requires California owners to register these wells; systems serving single-family homes need not register unless used by a home-based business.
Requirements vary by city and county: Phase I areas (larger cities and counties) and Central Coast agencies follow Regional Water Board rules, and local manuals set design storms. A July 2026 draft Small MS4 permit is not adopted as of September 2026.
Last reviewed .
Stormwater calculations for drainage facilities are civil engineering under California law (Business and Professions Code §6731). The calculations must be prepared by, or under the responsible charge of, a licensed civil engineer, who signs and seals the final report (§6735). Licensed architects practicing architecture are exempt from engineering registration (§6737), and small-project applicants use a State Water Board calculator to quantify site design measures.
LID calculations are needed when a project crosses the threshold in its municipal stormwater permit. Under the statewide Small MS4 permit, that means 5,000 square feet or more of new or replaced impervious surface. Detached single-family homes that are not part of a larger plan of development are excluded but need site design measures from 2,500 square feet. Phase I permits and Central Coast agencies set their own thresholds.
Stormwater calculations use measured infiltration rates wherever water soaks in, but the testing is a separate field task. It is done at trench or basin depth by a method and professional the agency accepts, often a geotechnical engineer. The proposal states whether testing is included.
The cost and schedule of stormwater calculations depend on site size, the number of drainage areas and off-site run-on, the agency's manual, which requirements apply (LID, peak-flow control or flood routing), infiltration testing and plan-check rounds. Share the site plan, topographic survey, soils report and agency comments; the proposal then sets scope and timing.